v3.26.3
Summary of Significant Accounting Policies (Narrative) (Details)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2026
USD ($)
Sep. 30, 2025
USD ($)
Jun. 30, 2025
USD ($)
Sep. 30, 2026
USD ($)
mi²
customer
segment
Sep. 30, 2025
USD ($)
Dec. 31, 2025
USD ($)
Apr. 04, 2025
USD ($)
Dec. 31, 2024
USD ($)
Significant Accounting Policies [Line Items]                
Number of reportable segments | segment       1        
Cash and Cash Equivalent $ 106.9     $ 106.9   $ 82.9    
Restricted Cash 0.1     0.1   0.1    
Excise taxes collected 14.4 $ 13.7   38.4 $ 38.0      
Customer receivables 152.3     152.3   117.3    
Unbilled Revenue 31.0     31.0   40.9    
Accounts Receivable, Allowance for Credit Loss 9.6 8.9   9.6 8.9 8.8   $ 6.1
Accounts Receivable, after Allowance for Credit Loss           161.1    
Accounts Receivable, Allowance for Credit Loss, Current 9.6     9.6        
Accounts Receivable, Credit Loss Expense (Reversal)       11.7 8.9      
Accounts Receivable, Allowance for Credit Loss, Writeoff       (11.0) (6.1)      
Accounts Receivable, Allowance for Credit Loss, Recovery       0.1 0.0      
Interest expense (19.5) (19.2)   (58.8) (63.0)      
Income Tax Expense (Benefit) (12.7) (11.5)   (28.0) (15.5)      
NET INCOME 38.6 13.4   85.2 18.4      
Nonoperating Income (Expense) 19.1 19.3   56.8 62.7      
Accumulated other comprehensive income 15.0     15.0   15.7    
Other Comprehensive Income (Loss), Net of Tax (0.1) (0.1)   (0.3) (0.1)      
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax       0.7        
Contract with Customer, Asset, before Allowance for Credit Loss 183.3     183.3   158.2    
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents 107.0 109.7   107.0 109.7 83.0   $ 54.1
Accounts Receivable, after Allowance for Credit Loss, Current 177.7     177.7   161.1    
Other Comprehensive (Income) Loss, Defined Benefit Plan, Reclassification Adjustment from AOCI, Tax (0.2) (0.2)   (0.4) (0.1)      
Allowance for Funds Used During Construction, Equity Portion 1.6 0.2   3.5 0.3      
Allowance for Funds Used During Construction, Capitalized Interest (1.1) (1.5)   (2.5) (4.4)      
Unrealized Gain (Loss) on Investments (0.1) $ 0.4   0.1 $ 0.6      
CDPQ Investment total             $ 544.0  
Property, Plant, and Equipment [Line Items]                
Property, Plant, and Equipment, after Accumulated Depreciation, Depletion, and Amortization 3,162.5     3,162.5   2,885.8    
Property, Plant, and Equipment, Accumulated Depreciation, Depletion, and Amortization (691.7)     (691.7)   (647.3)    
Contributions in Aid of Construction 58.0     58.0   27.0    
AOCI Attributable to Parent [Member]                
Significant Accounting Policies [Line Items]                
Other Comprehensive Income (Loss), before Tax, Portion Attributable to Parent     $ 17.5          
AOCI Attributable to Parent [Member]                
Significant Accounting Policies [Line Items]                
Other Comprehensive Income (Loss), before Tax, Portion Attributable to Parent     $ 2.7          
Nonrelated Party                
Significant Accounting Policies [Line Items]                
Other 2.2     2.2   10.0    
Related Party                
Significant Accounting Policies [Line Items]                
Other $ 1.8     $ 1.8   $ 1.7    
Subsidiaries [Member]                
Significant Accounting Policies [Line Items]                
Approximate number of retail customers | customer       546,000        
Service area, square miles | mi²       6,000        
CDPQ Investment percent             30.00%